Concrete can be one of the most durable building materials and structures made of concrete can have a long service life. Concrete is a composite construction material composed primarily of aggregate, cement, and water. It provides superior fire resistance, compared with wooden construction and can gain strength over time. Further, as it is used as liquid that subsequently hardens it can be formed into complex geometries and may poured either directly into formworks at the construction sites (so called ready mix concrete) or employed remotely to pre-build concrete elements and structures. Overall concrete is the most widely used construction material in the world with an annual consumption estimated at approximately 30 billion tons in 2006, compared to 2 billion in 1950. During the next 5 years concrete consumption is estimated to grow with a Compound Annual Growth Rate (CAGR) between 6% and 9% according to market forecasts of cement and concrete admixtures globally over the period 2012 to 2017 such that the 30 billion ton consumption will increase to approximately 40 billion tons.
Concrete technology was known by the Ancient Romans and was widely used within the Roman Empire, the Colosseum in Rome is largely built of concrete. After the Empire passed, use of concrete became scarce until the technology was re-pioneered in the mid-18th century with developments such as the method of producing Portland cement patented by Joseph Aspdin in 1824. There are many types of concrete available, created by varying the proportions of the main ingredients of cement, aggregate, and water as well as reinforcement means, chemical admixtures, and mineral admixtures. In this way or by substitution for the cemetitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.
Examples of chemical admixtures include accelerators to speed up the hardening of concrete, retarders to slow the hardening of concrete for large or difficult pours, air entrainments to capture air bubbles, plasticizers to increase workability, pigments for colour, corrosion inhibitors, bonding agents and pumping aids. Recently the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing the amount of cement required.
Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, motorways/roads, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats. Within the United States (US) alone concrete powers a US$35 billion construction industry, employing more than two million workers. More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete, pre-stressed concrete and precast concrete are the most widely used types of concrete functional extensions.
Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding steel reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads. Thereafter the concrete is reinforced to withstand the tensile loads upon it. Due to their low cost and wide availability steel reinforcing bar (commonly referred to as rebar) has been the dominant reinforcing material for the past 50 years. However, these steel rebars may corrode whereby the oxidation products (rust) expand and tend to flake, thereby cracking the concrete and reducing the bonding between the rebar and the concrete. Such corrosion may arise from several sources including carbonation when the surface of concrete is exposed to high concentration of carbon dioxide or chlorides, such as when the concrete structure is in contact with a chloride-contaminated environment such as arises with de-icing salts and marine environment.
Chlorides, including sodium chloride, contribute to the initiation of corrosion in embedded steel rebar if present in sufficiently high concentration. Chloride anions induce both localized corrosion (pitting corrosion) and generalized corrosion of steel reinforcements. Accordingly, the quality of water used for mixing concrete becomes important, as does ensuring that the coarse and fine aggregates do not contain chlorides, and nor do any admixtures contain chlorides. However, it was once common for calcium chloride to be used as an admixture to promote rapid setting of the concrete as it was also mistakenly believed to prevent freezing. However, this practice has fallen into disfavor once the deleterious effects of chlorides became known but a significant portion of existing concrete infrastructure employed calcium chloride. Additionally, the use of de-icing salts on roadways, used to reduce the freezing point of water, probably to date has been one of the primary causes of premature failure of reinforced or pre-stressed concrete bridge decks, roadways, and parking garages.
US bridges have been typically built to last 50 years. However, the average bridge in the US is now 47 years old. According to the U.S. Department of Transportation, of the 600,905 bridges across the country as of December 2008, 72,868 (12.1%) were categorized as structurally deficient and 89,024 (14.8%) were categorized as functionally obsolete. A structurally deficient bridge may be closed or restrict traffic in accordance with weight limits because of limited structural capacity. These bridges are not unsafe, but must post limits for speed and weight. A functionally obsolete bridge has older design features and geometrics, and though not unsafe, cannot accommodate current traffic volumes, vehicle sizes, and weights. These restrictions not only contribute to traffic congestion, they also cause such major inconveniences as forcing emergency vehicles to take lengthy detours and lengthening the routes of school buses.
With truck miles nearly doubling over the past 20 years and many trucks carrying heavier loads, the spike in traffic is a significant factor in the deterioration of bridges. Of the more than 3 trillion vehicle miles of travel over bridges each year, approximately 223 billion miles come from trucks. Accordingly, with the legal maximum weight of truck being 40 tons compared to an average car of 2 tons trucks account for approximately 9 trillion ton-miles of loading to bridges whilst cars account for approximately 5.5 trillion ton-miles.
Whilst road and railway bridges are highly visible occurrences of structural degradations from corrosion of rebar concrete structures these reinforced structures form the basis of common building foundations, buildings, footbridges, sewage systems, etc. Accordingly, determining corrosion within rebar concrete structures has been a focus of research and development for many years and issued testing standards with particular emphasis on electrical resistivity measurements within the laboratory from samples taken from structures.
Corrosion is an electro-chemical process. Accordingly, the flow rate of the ions between the anode and cathode areas, and therefore the rate at which corrosion can occur, is affected by the resistivity of the concrete. Empirical tests comparing electrical resistivity measurements with other physical and chemical analysis have generated the threshold values given by Equations (1) through (3) below as determining the likelihood of corrosion.
ρ > 120Ω · mcorrosion is unlikely(1)80Ω · m ≦ ρ ≦ 120Ω · mcorrosion is possible(2)ρ < 80Ω · mcorrosion is fairly certain(3)
These values have to be used cautiously as there is strong evidence that chloride diffusion and surface electrical resistivity is dependent on other factors such as mix composition and age. Further, the electrical resistivity of the concrete cover layer decreases due to increasing concrete water content, increasing concrete porosity, increasing temperature, increasing chloride content, and decreasing carbonation depth. However, as an overall industry rule when the electrical resistivity of the concrete is low, the rate of corrosion increases. When the electrical resistivity is high, e.g. in case of dry and carbonated concrete, the rate of corrosion decreases.
Laboratory based measurements of electrical resistivity may exploit two electrode methods wherein the concrete electrical resistance is measured by applying a current using two electrodes attached to the ends of a uniform cross-section specimen and electrical resistivity calculated. This method suffers from the disadvantage that contact resistance can significantly add to the measured resistance causing inaccuracy. Accordingly, this can be overcome by using four electrodes wherein a pair of outer electrodes is used to inject current as before, but the voltage is measured between a second pair of inner electrodes. The effective length of the sample being measured is the distance between the two inner electrodes. Less commonly employed is a transformer to measure resistivity without any direct contact with the specimen. The transformer consists of a primary coil which energises the circuit with an AC voltage and a secondary which is formed by a toroid of the concrete sample.
On-site electrical resistivity of concrete is commonly measured using four probes in a Wenner array which is used for the same reason as in the laboratory methods, namely to overcome contact errors. In this method four equally spaced probes are applied to the specimen in a line. The two outer probes induce the current to the specimen and the two inner electrodes measure the resulting potential drop. The probes are all applied to the same surface of the specimen and the method is consequently suitable for measuring the resistivity of bulk concrete in situ.
However, it would be evident that 600,000 concrete bridges with their associated support piers together with 55,000 miles of concrete road surface and billions of tons of concrete in buildings represent a significant measurement hurdle in terms of establishing protocols for rapid testing as well as associating the measurements specifically to particular elements of the physical infrastructure being evaluated. Accordingly, it would be beneficial for a field characterization system to automatically triangulate the location of the electrical resistivity device so that mapping of a structure can be performed without requiring an initial mapping of the structure to define measurement locations. It would be evident that erroneous association of electrical resistivity measurements to the wrong section of a structure may result in substantial disruption, such as closing the wrong side of a bridge to perform repairs where it then becomes evident the other side was actually corroding as the repairs having destroyed the road surface to get to the rebars find them non-corroded. Further, such erroneous activities substantially increase the overall costs of performing repairs straining already limited Federal and State budgets for example.
In other circumstances the concrete may have been covered with asphalt as a result of road resurfacing, repairs, etc. Accordingly, there is the problem of making quick and reproducible contact to the concrete through these overlying materials. It would therefore be beneficial to provide a means of improving this contact in such a manner. Likewise, it is the low frequency impedance of rebar in concrete that is correlated to the corrosion state of the steel reinforcement rods within the concrete. Accordingly, the direct measurement of the low frequency impedance of the rebar is a very time consuming measurement and one that is vulnerable to noise. As such, this low frequency technique is not easy to use in the field which is why commercial prior art electrical resistivity meters employ AC measurements of electrical resistivity at certain high enough frequencies. Hence, it would be beneficial to provide a means of making the electrical resistivity measurements that allows the low frequency resistivity to be derived from the measurements thereby improving determination of corrosion whilst reducing measurement times.
Likewise, prior art techniques for measuring the electrical resistivity of rebar, such as half-cell potential measurements, require that electrical connection is made to the rebar in contrast to concrete electrical resistivity measurements that determine the properties of the concrete surrounding the rebar. Accordingly, this requirement increases the complexity of making the measurements and requiring additional disruption/repair/cost even when no corrosion is identified. However, in many instances this is not feasible such as with epoxy coated steel rebar which is intended to reduce the occurrences of corrosion but as the rebars are electrically isolated from each other half-cell potential measurements are infeasible. As such it would be beneficial to provide a method of determining the state of rebar without requiring an electrical contact to the rebar with in the concrete infrastructure.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.